Catalytic reforming of naphtha plays an important part in increasing octane number and producing aromatic hydrocarbons. Recently, reforming technology and the catalyst therefor have been improved continuously and the developments in improving the catalyst performances are remarkable. Specifically, Pt-Re catalyst changes the stability of the catalyst itself considerably. In 1980s, it was pointed out that reaction performances of the catalyst containing higher content of Re (Re/Pt=2-3) were better than that of the conventional catalyst (Re/Pt=1).
Moreover, the carrier has been continually improved together with the introduction of metal Re component and the adjustment of Re/Pt ratio. Among reforming catalyst gamma-alumina exhibits excellent performances. More specifically, gamma-alumina carrier modified by various methods has improved the reaction performances of the catalyst in varying degrees. Proper modification of the gamma-alumina carrier can not only improve the performances of the catalyst, but also decrease the content of noble metal Pt, so that the cost is reduced.
The alumina differs greatly in performances owing to different methods of preparation and treatment, thus exerting great influence on catalyst performances in application. As exemplified in U.S. Pat. No. 4,178,268, SB alumina which is a byproduct from Ziegler synthesis reaction is used as a support to prepare reforming catalysts which exhibit a good reactivity. DE Pat. 3,508,746 introduces a reforming catalyst using alumina as a support which is prepared by hydrolyzing aluminum alkyl produced by the reaction between Al and C.sub.m --OH(m&gt;4). Chinese Pat. 87107556.3 introduces another highly selective and active catalyst using HP alumina as a support which is prepared by the method exemplified in Chinese Pat. 85100218. In brief, the above-mentioned catalysts are prepared through different ways and disposal methods to form different gamma-alumina supports, so as to enhance the reaction performances of the catalyst. Generally, above-mentioned gamma-alumina are prepared by continually disposing dry aluminum hydroxide powders or wet filtered cake: acidification with nitric acid or other acids at room temperature, forming, drying and calcining. Thus prepared gamma-alumina has a poor crystallinity, small porosity, and larger proportion of small pores, especially, poor thermal stability. It is inevitable that the reaction performances, e.g. activity, stability, especially the selectivity of the catalyst appear to be unsatisfied.
The lowest content of Pt of the reforming catalyst used in industry is above 0.2 Wt. %, usually 0.22-0.3 Wt. % in order to assure its reactivity. In addition, the selectivity of the currently used catalyst with high Re/Pt ratio (Re/Pt&gt;2.0) is lower than that of conventional catalyst (Re/Pt=1). The yield of reforming liquid and the concentration of hydrogen are low at initial stage, therefore, it can only be used in the last reactor of industrial reforming equipment to prevent the yield of liquid products from excessive losing.
The object of this invention is to provide a suitable method to overcome the deficiency of the prior arts and to prepare the gamma-alumina with high crystallinity, proper bulk density and concentrated middle pores so as to yield a highly selective, active and stabilized Pt/Re reforming catalyst containing less noble metals. Specifically, a catalyst of better performances with higher Re/Rt ratio (Re/Pt=3) has been prepared.